1. Field of the Invention
This invention relates generally to the cell structure, device configuration and fabrication process of power semiconductor devices. More particularly, this invention relates to an improved cell configuration and processes to manufacture trench MOSFET device with junction barrier Schottky rectifier in the same cell so that integrated cells with spacing savings and lower capacitance and higher performance are achieved.
2. The Prior Arts
Normally for high efficiency DC/DC application, a Schottky rectifier is externally added in parallel with a MOSFET device. FIG. 1 is a circuit diagram that illustrates the implementation of a Schottky diode with a power MOSFET device. Once the parasitic P/N diode is turned on, both the electron and hole carriers are generated thus require longer time to eliminate the carriers by electron-hole combination. In order to achieve higher speed and efficiency, the Schottky diode (SKY) is connected in parallel to the MOSFET device with the parasitic PN body diode to function as a clamping diode to prevent the body diode from turning on. The Schottky Diode is single carrier, i.e., electron carrier only and that can be drawn simply by the drain Electrode. The requirement for the clamping effect is that the Forward Voltage of the Schottky diode Vf is less than the parasitic PN diode (˜0.7V). As the electronic devices become more miniaturized, there is requirement to integrate the Schottky diode as part of the semiconductor power device as an IC chip to reduce the space occupied by the Schottky diode instead of connecting the Schottky diode as an external electronic component.
In U.S. Pat. No. 6,351,018, a trenched MOSFET device integrated with trench Schottky diodes with common trench gates is disclosed as that shown in FIG. 2. In U.S. Pat. No. 6,593,620 discloses another trench MOSFET device with a trench Schottky diode with separated trench gates as shown in FIG. 3. In U.S. Pat. No. 6,987,305 (not shown) discloses another trench MOSFET device which is similar to U.S. Pat. No. 6,593,620 except thick gate oxide on trench bottom. The configurations as disclosed in the patented invention have a disadvantage that the Schottky diodes occupy additional space for planar contact that is about the same space as the MOSFET. The Trench Schottky diodes further suffer from a high leakage between drain and source due to phosphorus increase at channel region during the sacrificial and gate oxidation processes. Furthermore, the device as shown has a higher capacitance due to the presence of the trench MOS-Schottky structure which has inherent parasitic capacitance from trench sidewall and bottom in trench MOS-Schottky.
In U.S. Pat. No. 6,433,396, a trench MOSFET device with a planar Schottky diode is disclosed as that shown in FIG. 4. The configuration again has disadvantages that the Schottky diode occupies additional space for planar contact and reverse leakage current Ir between anode and cathode is high. Also, the formation process requires additional contact mask for the Schottky diode thus increases the cost and processes complications for producing the MOSFET power device with Schottky diode.
In U.S. Pat. No. 6,998,678 discloses another trench semiconductor arrangement as shown in FIG. 5 with a MOS transistor which has a gate electrode, arranged in a trench running in the vertical direction of a semiconductor body, and a Schottky diode which is connected in parallel with a drain-source path (D-S) and is formed by a Schottky contact between a source electrode and the semiconductor body The configuration has disadvantage that it is difficult to optimize both performance of the Schottky diode and the trench MOSFET when they share same mesa space between two adjacent trenches and same source trench contact. Furthermore, the manufacturing cost is increased due to the requirement that an additional P+ mask is required to form the trench Schottky diodes.
Therefore, there is still a need in the art of the semiconductor device fabrication, particularly for design and fabrication of the trenched power device, to provide a novel cell structure, device configuration and fabrication process that would resolve these difficulties and design limitations. Specifically, it is desirable to provide more integrated trench MOSFET with embedded Schottky diode that can accomplish space saving, process simplification and capacitance reduction such that the above discussed technical limitations can be resolved.